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Monday, July 16, 2012

Nature, nurture and the control of food intake

Some weeks ago,
my blog was entitled: “Ever seen a fat fox”, the gist of which was that whereas
biology can tell us a lot about the control of food intake in animals, in man,
with a large pre-frontal cortex, living in a highly social existence, social
aspects may be far more important than the biology. I return to this theme
today in light of some intriguing research jointly carried out by researchers
at the Universities of Washington and Toronto[1].
The research centers around the phenomenon of restrained eating, so let me
first explain what this means. As I have previously pointed out, overweight and
obesity are not usually the outcome of any conscious decision to get fat (Sumo
wrestlers excluded). Restrained eating, on the other hand, is a very conscious
decision to do, as the name implies, to consciously count calories in order not
to gain weight or to maintain weight loss. Restrained eaters frequently have
experienced some weight gain and have then made the lifetime commitment to
being a fussy eater.

We know that
obesity is highly heritable and based on studies of identical and non-identical
twins, the figure for heritability is as high as 90%. So, it seemed a
reasonable question to ask if brain function in response to food cues in restrained
eating also had a similar level of genetic control. Thus the researchers used
the University of Washington’s register of twins. In 2006, the twins took part
in a health survey that involved the completion of a restraint-eating
questionnaire (10 questions), effectively examining the concerns of the
subjects about chronic dieting and weight gain. The researchers then mined the
database to identify sets of identical twins that differed radically in their
restrained eating patterns. Effectively, each twin pair had one serious
restrained eater and one with no restraint as regards eating. By using
identical twins, the study immediately eliminated any biological difference,
since identical twins are genetic clones. The participants were all female and
in their early thirties. The twins were shown photographs of “fattening” and
“non-fattening” foods and then underwent a brain scan using a system known a
functional Magnetic Resonance Imaging (fMRI). This was then repeated after the
volunteers had consumed a milk shake.

The first test
showed that the twin with the restrained eating personality showed much higher
brain activity following exposure to pictures of high-calorie “fattening” foods
than the twin with no tendency to food restraint. Specifically, the areas of
brain activity most affected were the amygdala (involved in emotional
processing of external cues), the occipital lobe (involved in behaviour
modification) and in the right thalamus (involved in visual perception).
Interestingly, there was no difference between restrained-eaters and non-restrained
eaters in the prefrontal cortex. This is somewhat surprising to a
non-neurobiologist such as myself since this is the area of the brain
associated with all the higher complex brainpower of humans and since humans
are the only species that exhibits restrained eating, one could be forgiven for
thinking that the pre-frontal cortex would be involved. Things now get complex
after the subjects had consumed the milkshake. In the case of the restrained
eaters, all those areas of enhanced activity seen in the first test with
pictures of high calorie “fattening” foods were greatly reduced. Strangely
however, the sight of the non-fattening foods now switched on several parts of
the brain, which was not observed in the non-restrained twins.

The authors point
out that the fMRI data are consistent with other data, which show that
restrained eaters are highly sensitive to external cues, which, in this study
led to enhanced brain activity in the three regions mentioned. They see this as
a conflict between food appeal and the desire to restrain eating. After the
milkshake, this enhanced activity diminishes which the authors say may have
been “cognitively driven”. I take that to mean that the intake of the milkshake
“woke them up” in a sense such as to inhibit their natural desires.

For me the big
deal in this paper is that an acquired habit regarding the regulation of food
intake, restrained eating, is not entirely genetically driven. Moreover, it is
not associated with the pre-frontal cortex, which makes man man and mice mice. Previous
studies by the same authors in a very large twin study found that restrained
eating was partly heritable with a range of 35% to 50% heritability recorded in
95% of the subjects. So some element of restrained eating is inherited and
another, learned.Such is the
utter complexity of the regulation of food intake in humans. So, the next time
you are at a conference and someone starts to discuss the regulation of food
intake in rats and mice, take out the smart phone and do your e-mails!

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"Ever seen a fat fox ~ Human obesity explored"

About Me

I graduated from University College Dublin in 1971 with an Masters in Agricultural Chemistry, took a PhD at Sydney University in 1976 and joined the University of Southampton Medical School as a lecturer in human nutrition in 1977. In 1984 I returned to Ireland to take up a post at the Department of Clinical Medicine Trinity College Dublin and was appointed as professor of human nutrition. In 2006 I left Trinity and moved to University College Dublin as Director of the UCD Institute of Food and Health. I am a former President of the Nutrition Society and I've served on several EU and UN committees on nutrition and Health. I have published over 350+ peer reviewed scientific papers in Public Health Nutrition and Molecular Nutrition and am principal investigator on several national and EU projects (www.ucd.ie/jingo; www.food4me.org). My popular books are "Something to chew on ~ challenging controversies in human nutrition" and "Ever seen a fat fox: human obesity explored"